1.Field of the invention
The present invention concerns the synchronous transmission of data at very high bit rates, in particular over optical fiber links.
2.Description of the prior art
On a data transmission link, the data is transmitted in the form of a sequence of isochronous symbols having zero DC energy content and little energy content at low frequencies in order to simplify monitoring the level of the transmission sources and the gain of the receivers, to simplify equalization and to enable transmission over the same medium of service channels and remote supervisory channels processed by separate equipment in the terminals and at intermediate regeneration points. This sequence of symbols is produced from the data using a redundant code providing for the transmission of symbol timing information needed for their recovery and transmission quality information.
In the case of optical fiber links, use is preferably made of a binary type two-level line signal because of non-linearities of the optical source and its temperature dependence. The binary-binary code used is generally an nBmB type block code with multiple alphabets and a limited digital sum which converts blocks of n bits into blocks of m bits, m being greater than n, in such a way that the difference between the numbers of ones or marks and zeros or spaces transmitted is null on average. This type of redundant code does not utilize all possible configurations of the blocks of m bits. It enables those least favorable to timing recovery to be eliminated and provides for the reservation of certain configurations which cannot be imitated by a succession of bits belonging to two consecutive blocks of m bits authorized for use to synchronize the blocks. It further provides for detecting line errors by monitoring the appearance of unauthorized configurations of blocks of m bits at the receiving end. Unfortunately, its use entails somewhat complex manipulation of the bits which is difficult to implement for very high bit rates, since the limiting operating speeds of modern semiconductor technologies are too low.
In order to circumvent this problem there has already been proposed a block code entailing dividing the data to be transmitted into consecutive blocks of M bits and coding each of the blocks by a word of (M+1) bits consisting of the M bits of the block complemented or not according to whether the encoded word on (M+1) bits which would be obtained without complementing features a disparity (the difference between the numbers of ones and of zeros) of the same sign or the opposite sign to that of the set of symbols already transmitted, to which is added a complement bit placed at the beginning of each block to indicate whether or not complementing has been applied. This type MB 1C code, as compared with multiple alphabet nBmB block codes, has the disadvantage of not permitting block synchronization recovery or error detection. One way of circumventing the block synchronization problem consists in adding to the beginning of each block of M bits already provided with its complement bit, prior to complementing (if applied) and transmission, a frame bit with the same value as the last bit of the block. This MB 1C 1F type code does not overcome the problem of detecting errors, which must be done by previous processing of the data using an error detection code and reduces the overall efficiency of transmission.
An objective of the present invention is a method for synchronous transmission of data using a code of the aforementioned MB 1C 1F type which further provides for error detection and which is easier to implement for high transmission bit rates than multiple alphabet nBmB type codes.